PROBLEM
Case 1
Capture One freezes when the files are imported from the folder which name has German special characters such as ä, ö, ü, or ß.
Case 2
I cannot create a Catalog/Session with the name containing language-specific characters (ä, ö, ü, á, é) and special symbols.
Case 3
After exporting all the pictures, I have noticed that special German letters are displayed wrong.
Case 4
I use the pipe symbol "|" (the vertical bar) in my folder naming. When attempting to open a Session within a folder with that symbol, the previews won't load.
SOLUTION
In general, Capture One makes a warning when invalid characters are used for naming Catalogs/Sessions or User Collections (Albums, Smart Albums, Groups, Projects). Also, when the keywords with special symbols are imported from the keyword library file, the warning message would be displayed.
However, when it concerns the folders and files outside Capture One, some weird behavior might be expected as file paths containing special characters are not always processed correctly.
When naming folders from which the images will be imported or to which they would be exported, it would be a good practice to use only the Latin characters and digits from the basic ASCII set. Also, please avoid the pipe character "|" (vertical bar), / (on Mac), \ on Windows, and any symbol listed in the extended ASCII table. Otherwise, the paths to and from those folders would not be operated correctly. The previews might not be rendered appropriately as well. Moreover, software crash or another kind of unexpected behavior might occur when the file paths contain special characters.
When naming or renaming the files or folders, it is also recommended to avoid using the .(dot) character.